Second choice is in, the new bard is the "Emmisary". Seems we're going to keep using one-word names, to fit with the other classes. I was not aware this was a conscious design decision on my partner's part, or I'd have said something sooner. Third choice, the cosmic theurge will now just be "Theurge", meaning only the mortal theurge needs a new name.

Harbinger

I would personally make cosmic theurge Harbinger and let mortal theurge just be theurge.

You said cosmic theurge's are "Mages who hear voices in their minds, which they believe to be of higher beings. The louder these voices grow (called "insight points"), the more accurate their spells become but the lower their defences get. Insight is also built by some spells, all wildly unpredictable, and spent by others, all horrifyingly precise"

To me that kind of says they are communicating with some sort of madness or extra worldly thing that "sane" people aren't aware of.

There description makes me thing that these voices are using the cosmic theurges to enact their will upon the world. Making them into the ones who will tell others of their coming.

With this being the case (in my mind at least). Cosmic theurges would be more deserving of the title of Harbinger of ****
while mortal theurges could then be comfortably just referred to as theurges.
Makes me think of the cthulhu mythos and stuff. Prophets who warn of the things that lurk outside time and space, yet also draw power from it.

If I may ask, are you developing stuff for a world your building or creating your own game system or something?

Both. It's a new game system and it has two settings attached to it, Aelsif and a space fantasy my partner's been intentionally excluding me from. All of these names are for character classes used in Aelsif, but I don't know if any classes are shared by the other setting.